Meta-Biol

• Pathways

Kynurenine-oxoglutarate transaminase 1 (KYAT1, aka CCBL1, KAT1) catalyzes the reaction of kynurenine (L-KYN) and pyruvate (PYR) to form 4-(2-aminophenyl)-2,4-dioxobutanoate (AP-DOBu) and alanine (L-Ala). The active form of KYAT1 is a homodimer with one molecule of pyridoxal phosphate (PXLP) bound to each monomer (Baran et al. 1994, Han et al. 2009, Rossi et al. 2004). The enzyme's cytosolic localization is inferred from recombinant protein overexpressed in transfected cells (Perry et al. 1995). The pH optimum observed for KYAT1 in vitro is 9.5 - 10.0, so its role in kynurenine metabolism in vivo is not clear (Baran et al. 1994).

Biochemical studies of KYAT1 activity in vitro (e.g. Baren et al. 1994) invariably measure kynurenic acid as the reaction product, not AP-DOBu, the product to be expected from transamination of kynurenine. The condensation of AP-DOBu and elimination of a water molecule to form kynurenic acid has not been demonstrated directly. As noted by Miller et al. (1953) discussing their characterization of a bacterial form of the enzyme, "The keto acid assumed to be formed prior to ring closure in the conversion of kynurenine to kynurenic acid has not yet been detected. In principle, such detection should be possible, since it is sufficiently stable to have been synthesized. It also remains to be established whether ring closure is spontaneous, enzymatic, or both. The formation of kynurenic acid from L-kynurenine by the L-amino acid oxidase of Neurospora suggests, however, that ring closure can be spontaneous, unless the somewhat improbable assumption is made that Neurospora filtrate contained the ring-closing enzyme."

The alpha keto acids indole-3-propionic acid (I3PROPA) and indole-3-lactic acid (I3LACT) are potent inhibitors of KYAT1 (Han et al. 2009). Phenylalanine is an effective competitive inhibitor of kinurenine aminotransferase 1 (Han et al. 2004).